$ apt-cache search libxfconf
$
Apparently, it's not in my /etc/apt/sources.list
Where can I find it?

In sid.

OK, if I upgrade, will that give me sid or do I have to install it?

I'd tell apt to catch sid and dist-upgrade.

How do I "catch sid"? I never heard of that phrase before.

Hi Shawn

Beware, thar be dragons :-)

That you're asking these questions suggests it is a bit early for you
to consider upgrading to sid (he's the one in Toy Story who breaks toys
in spectacular fashion) aka unstable.

I guess you're running lenny/stable and I've just checked the apt-cache
one of our ppc servers here and yes, it's not available in Lenny.

You may not need to upgrade the whole system to sid but you may be able
to upgrade that package alone or more probably, xfce.

Before you do anything I would install apt-listbugs because when you're
upgrading stuff it will show you what's got bugs.

After that edit /etc/apt/sources.list and change all references from
lenny or stable to sid, then save the file and run either:
#apt-get update
or
#aptitude update
DO NOT upgrade

If you are familiar with aptitude in interactive mode search for the
package and try to upgrade just that package. Aptitude will show you
if you're going to break something and hitting "e" will offer various
solutions. Alternatively:
# apt-cache search libxfconf
will show you what package is available and then do:

#apt-get install packagename

It should notify you of missing dependencies etc.

If it does install without a hitch, go back to your sources.list and
change sid to lenny again. Do another apt-get update and all will be
back to normal. When you're a bit more familiar with Debian and
feeling adventurous, google on stable versus sid.